1. Field of the Invention
The present invention relates to an image forming apparatus, having an image formation device and a thermal fixer, and in particular to an image forming apparatus operable under different image formation conditions of various resolution levels and process velocities or the like.
2. Description of the Background Art
In various image forming apparatuses, such as copiers, printers, duplicators, etc., an image is borne on a recording medium, such as a transfer medium, a sheet like medium, etc., and is finally fixed onto a copying or printing sheet by a fixer. The fixer employs a pair of opposed rollers composed of a heating roller and a pressing roller that presses the recording medium against the heating roller to form a nip therebetween. The recording medium is pinched and conveyed through the nip, so that the image not yet fixed is fixed by heat generated by the heating roller as shown in FIG. 3.
Recently, as a printer increases its resolution, a high resolution mode, such as a 1,200 dpi (dot per inch) mode, etc., is demanded.
However, when the resolution is increased from 600 dpi to 1,200 dpi for example, a number of rotations of a polygon mirror, which is provided in a laser scanning system of a laser beam printer, generally needs to be increased to be doubled to more than 30,000 rpm (revolutions per minute), etc., for example, thereby posing technical and manufacturing cost problems.
To resolve such inconvenience, a conventional printer employs a process velocity changing system and changes a resolution level and a process velocity (equivalent to a conveyance velocity, a line velocity of a recording medium, and an image formation velocity or the like). For example, a conventional printer capable of outputting 22 sheets per minute with 600 dpi at a process velocity of 120 mm/sec halves the process velocity down to half to output 11 sheets per minute with 1,200 dpi.
In such a printer, since the number of rotations of a polygon mirror is kept constant regardless of the resolution, technical and manufacturing cost problems do not occur. However, when the line velocity is changed between 120 mm/sec and 60 mm/sec, an optimum value of a process condition for electro-photographic process condition changes, and accordingly a target fixing temperature of a fixer needs to be changed as well.
For example, when the line velocity is 120 mm/sec, the target fixing temperature is to be increased even when handling the same sheet size. By contrast, when it is 60 mm/sec, the target fixing temperature is to be decreased.
Further, a ratio of color images to total outputs from the image forming apparatus is increasing these days and glossiness of a fixed toner image is one of measures for evaluating the color image. An image having a low glossiness (generally equal to or less than 15%) is popular in typical business documents, whereas that having a high glossiness (equal to or more than 20%) is popular for photographs, catalogs, and brochures printing or the like as trend. Thus, depending on the usage of the output image, a prescribed image forming apparatus is selectively used.
Such a high glossiness image can be formed by increasing the target fixing temperature with an increased amount of toner attracted to a sheet. However, when such conditions are maintained, an ordinary image not requiring such high glossiness is undesirably and excessively supplied with toner and heat in the fixer in ecology and energy saving view points. Accordingly, the processing conditions need to be changed in accordance with the type of an image to be formed, while the target fixing temperature needs to be changed to form and fix an image with the minimum amount of both toner particles and heat.
However, as described above, when an image formation condition or a line velocity is changed, a target fixing temperature needs to be changed to be optimum.
As a result, since a passage time for a sheet to pass through a fixing nip increases when the image formation velocity is decreased, calorie is excessively applied to both a toner image and the recording medium itself bearing the toner image thereon. When temperature in the nip of the fixer is maintained to be high, hot offset likely occurs or an amount of curl of the recording medium increases. Accordingly, a surface temperature of the fixer needs to be decreased to a prescribed level before executing such a fixing process to avoid occurrence of such abnormalities.
However, since a calorific capacity of the fixer of FIG. 3 is relatively large, a certain waiting time is needed to decrease temperature of the fixer from high to low levels when the image formation line velocity is changed lower. It is also true when an image formation condition is changed even maintaining the image formation line velocity.
Japanese Patent Application Laid Open No. 07-306609 (JP-H07-306609-A) attempts to resolve such a problem and employs a system of forcibly cooling a heating roller using a blower module.
Specifically, the system of JP-07-306609-A appropriately changes a force of a blower when an image formation line velocity, and accordingly a target fixing temperature are changed. In particular, it increases the force when cooling thereof so as to minimize the waiting time. Japanese Patent Application Laid Open No. 2000-181275 (JP-2000-181275-A) also attempts to resolve the problem and additionally employs a controller to control a blowing force of a blower module for the same purpose.
However, in both of the above-described prior arts, since the cooling system cools the fixer down to a prescribed setting temperature, (excessive?) calorie of thermal storage is necessarily wasted in the fixer. In addition, since the blowing force of the blower is increased, and accordingly more energy is consumed in driving the cooling system, an image forming apparatus does not effectively save energy.